Mycoplasma pneumoniae is the causative agent of Atypical Primary Pneumoniae. Attachment of the organism to the respiratory epithelium is a prerequisite to infection. Krivan H.C. et al. (1989. J. Biol. Chem.) reported that M. pneumoniae binds avidly to sulfatides. A 52 kDa protein bound to the dextrane sulfate column, and a 52 kDa protein can be detected in membrane preparations from M. pneumoniae with monoclonal antibody CP3-46F5 (MAbF5). The goal of this study is to define the role the 52 kDa protein plays in adherence of M. pneumoniae. A small size Chromosomal DNA library from M. pneumoniae M129 was prepared to circumvent the termination of a message in Escherichia coli due to the UGA codon. M. pneumoniae uses UGA as a tryptophan codon. E. coli colonies expressing epitopes of the 52 kDa protein were detected with monoclonal antibody CP3-46F5. A 400 bp partial clone was used to identify and clone a 6.5 fragment from a chromosomal digest of M. pneumoniae. Surface proteolysis of M. pneumoniae cells indicated that the protein is surface exposed. Immune electron microscopy studies localized the protein to the tip adherence structure of the cell. Mb F5 inhibited adherence of M. pneumoniae to MRC-5 cells. Taken together, these findings indicated the importance of the protein in adherence of M. pneumoniae. Attachment of M. pneumoniae to the respiratory tract is followed by ciliostasis, and destruction of the mucosal epithelium. The nature of events leading to the destruction of the respiratory epithelium is not clear. Animals receiving the extract developed necrotizing pneumoniae with denudation of the epithelial lining. The role the protease extract plays in the destruction of the respiratory epithelium by M. pneumoniae is being studied. The protease was analyzed using protease inhibitors and substrates, and was characterized as a collagenase with specificity to collagen of the respiratory basement membrane. The collagenase is being purified from cell free extract of M. pneumoniae.